His exact intentions remain unclear but the US is reportedly considering a military response.
The BBC's North America editor says a US strike could be imminent.
US Secretary of State Rex Tillerson has said Bashar al-Assad should have no role in a future Syria.
His comments signal an apparent U-turn for the US - only last week the US ambassador to the UN, Nikki Haley, said Washington was no longer prioritising the removal of the Syrian president.
